A method called an MS/MS analysis (also called a tandem analysis) is known as one of the mass spectrometric analysis techniques for identification and structural analyses of substances having large molecular weights. A tandem quadrupole mass spectrometer (also called a triple quadrupole mass spectrometer) having a relatively simple and inexpensive structure is one of the widely used mass spectrometers capable of performing the MS/MS analysis.
As disclosed in Patent Literature 1, generally in the tandem quadrupole mass spectrometer, quadrupole mass filters are provided respectively before and after (i.e. at the front stage and rear stage of) a collision cell for dissociating ions, where precursor ions are selected by the front-stage quadrupole mass filter from among a variety of ions originating from a target compound, and product ions are separated by the rear-stage quadrupole mass filter in accordance with their mass-to-charge-ratios. The collision cell has a box-like, relatively tight-sealed structure, and a CID gas such as argon and nitrogen is introduced into the collision cell. The precursor ions selected by the front-stage quadrupole mass filter are introduced into the collision cell endowed with appropriate collision energy, and collide with the CID gas inside the collision cell. As a result, collision-induced dissociation occurs, and the product ions are produced.
The dissociation efficiency of ions inside the collision cell depends on the amount of collision energy of the ions, the CID gas pressure inside the collision cell, and the like. Hence, the detection sensitivity of the product ions that have passed through the rear-stage quadrupole mass filter also depends on the amount of collision energy and the CID gas pressure.
In the tandem quadrupole mass spectrometer, a measurement in a multiple reaction monitoring (MRM) mode is performed in many cases, in order to perform quantitative determination on a known compound with high accuracy and sensitivity. In the MRM measurement mode, for both the front-stage and rear-stage quadrupole mass filters, the mass-to-charge-ratios of the ions that pass through the filters are fixed. Hence, in conventional tandem quadrupole mass spectrometers, the CID gas pressure inside the collision cell is set to a value (normally, at several mTorr) that is determined in advance by a manufacturer such that the detection sensitivity is as high as possible in the MRM measurement mode. Of course, the CID gas supply pressure can be manually adjusted by a user, whereby a measurement can be performed with higher sensitivity than that under such a preset condition as described above, with regard to, for example, a specific compound.
In general, as the CID gas pressure inside the collision cell becomes higher, ions become more likely to contact the CID gas, and hence the dissociation efficiency of the ions becomes higher. However, the kinetic energy of the ions is attenuated by the collision with the CID gas, and hence the flight speed of the ions as a whole decreases. In the case of the MRM measurement mode, dissociation of precursor ions having a given mass-to-charge-ratio and selection and detection of product ions having a given mass-to-charge-ratio are performed for a certain amount of time, and hence the decrease in ion flight speed in the collision cell as described above is considered to have relatively small influences on the ion intensity. However, in actuality, even in the MRM measurement mode, if the CID gas pressure is raised, the ion intensity obviously decreases compared with the case where the CID gas pressure is low. As a result, there arises a problem that a sufficiently high peak cannot be obtained on a mass chromatogram at a mass-to-charge-ratio corresponding to a target compound and that the quantitative accuracy thus decreases.
Moreover, a wide variety of measurement modes other than the MRM measurement mode described above are prepared for the tandem quadrupole mass spectrometer. Examples of the other measurement modes include: measurement modes in which both the front-stage and rear-stage quadrupole mass filters perform ion selection (a precursor ion scan measurement mode, a product ion scan measurement mode, a neutral loss scan measurement mode, and the like); and measurement modes in which one of the front-stage and rear-stage quadrupole mass filters does not perform ion selection (namely, all ions pass through the filter intact) while the other of the front-stage and rear-stage quadrupole mass filters performs a mass scan. In general, in all measurement modes except for measurement modes in which CID is not performed inside the collision cell, the CID gas pressure inside the collision cell is set to a value that is determined in advance by the manufacturer such that the detection sensitivity is as high as possible in the MRM measurement mode, as described above.
However, under such control, in the precursor ion scan measurement mode and the neutral loss scan measurement mode in which the front-stage quadrupole mass filter performs a scan over a predetermined mass-to-charge-ratio range, a mass-to-charge-ratio deviation of a target ion peak on a mass spectrum (MS/MS spectrum), which results from the decrease in ion flight speed inside the collision cell as described above, tends to become large. The degree of the decrease in ion flight speed inside the collision cell also depends on the ion size (which normally corresponds to the mass-to-charge-ratio). Hence, the degree of the mass-to-charge-ratio deviation on the mass spectrum is not always constant, and it is not easy to obtain the amount of deviation and correct the deviation. Moreover, variation in flight speed increases even among ions having the same mass-to-charge-ratio, and this causes a problem that the peak width becomes large on the mass spectrum, resulting in a decrease in mass resolution.
Furthermore, the MRM measurement mode is used in many cases for a simultaneous multicomponent analysis by a liquid chromatograph mass spectrometer or a gas chromatograph mass spectrometer, and the number of pairs of a precursor ion and a product ion to be detected simultaneously in parallel increases if the number of measurement target compounds increases. To deal with this, it is necessary to make high-speed switching of the mass-to-charge-ratio of ions that are allowed to pass through the front-stage quadrupole mass filter. As a result, such influences of the decrease in ion intensity as described above become further remarkable. On the other hand, even though the number of measurement target compounds is decreased in order to perform a measurement on each compound with high sensitivity, the improvement in sensitivity is limited.